Smart pixel arrays (SPAs) are devices containing arrays of vertical cavity surface emitting lasers (VCSELs) and photodetectors. SPAs are capable of performing high-speed switching and routing of digital data. The increased capabilities of SPAs require increased switching speeds and low bit error rate. This in turn requires the interconnections between devices to have low parasitic capacitance. Otherwise, the capacitance reduces switching speed and increases the bit error rate due to noise integrated on the photodetector.
Normally, the devices making up a SPA have anode and cathode contacts, one on the top and one on the bottom of the device. However, for practical low cost assembly, it is preferable to have both contacts on one side (usually the non-emitting side) of the wafer. This enables the SPA to be easily flip-chip bonded to the integrated circuit that interfaces with the SPA.
Thus, it is necessary to use a through-wafer via to bring the one contact to the opposite side of the device. This configuration may be the largest contributor to parasitic capacitance, due to the proximity of the signal line to the common substrate. The parasitic capacitance of the structure is substantial because of the large surface area of the anode pad. However, the size of the anode pad cannot be reduced without compromising yield of the flip-chip interconnect process. To counteract this problem, protons are implanted between the VCSEL devices. Although this reduces the parasitic capacitance, it does not eliminate it or reduce it to an acceptable level.
The structure of SPAs also generally require the anode of the VCSEL to be driven instead of the cathode because the cathodes are common to all of the VCSELs when using the conventional N-type substrate. VCSELs driven by their anodes are undesirable because it requires use of slower P-channel transistors. Therefore, it is desirable to have a SPA structure in which the anode is on the same surface as the cathode and neither anode nor cathode is electrically common to the substrate.
Devices with both sources and detectors can often suffer from “cross-talk”. This creates an undesirable situation where, for example, the source can alter the detected response and thereby change the perceived signal. This problem can be solved by electrically isolating the source and detector. Therefore, there exists a need for a device that is capable of electrically isolating multiple components with anode and cathode on the same surface while still maintaining low parasitic capacitance.